I. Field
The present invention relates generally to data communication, and more specifically to techniques for performing channel estimation with reduced complexity.
II. Background
Wireless communication systems are widely deployed to provide various types of communication such as voice, packet data, and so on. These systems may be multiple-access systems capable of supporting communication with multiple users by sharing the available system resources. Examples of such multiple-access systems include code division multiple access (CDMA) systems, time division multiple access (TDMA) systems, and orthogonal frequency division multiple access (OFDMA) systems.
Orthogonal frequency division multiplexing (OFDM) effectively partitions the overall system bandwidth into a number of (N) orthogonal subbands. These subbands are also referred to as tones, frequency bins, and frequency subchannels. With OFDM, each subband is associated with a respective subcarrier upon which data may be modulated. Each subband may thus be viewed as an independent transmission channel that may be used to transmit data.
In a wireless-communication system, an RF modulated signal from a transmitter may reach a receiver via a number of propagation paths. For an OFDM system, the N subbands may experience different effective channels due to different effects of fading and multipath and may consequently be associated with different complex channel gains.
An accurate estimate of the response of the wireless channel between the transmitter and the receiver is normally needed in order to effectively transmit data on the available subbands. Channel estimation is typically performed by sending a pilot from the transmitter and measuring the pilot at the receiver. Since the pilot is made up of symbols that are known a priori by the receiver, the channel response can be estimated as the ratio of the received pilot symbol over the transmitted pilot symbol for each subband used for pilot transmission.
Pilot transmission represents overhead in a wireless communication system. Thus, it is desirable to minimize pilot transmission to the extent possible. However, because of noise and other artifacts in the wireless channel, a sufficient amount of pilot needs to be transmitted in order for the receiver to obtain a reasonably accurate estimate of the channel response. Moreover, the pilot transmission needs to be repeated to account for variations in the channel over time due to fading and changes in the multipath constituents. Consequently, channel estimation normally consumes a noticeable portion of the system resources.
In an OFDM system, to reduce the amount of overhead for pilot, a pilot transmission may be sent on a group of designated subbands, which may be only a subset of the available subbands. An initial estimate of the channel response may be obtained for the designated subbands based on the pilot transmission. Signal processing may then be performed to obtain an enhanced channel response for a group of desired subbands, which typically includes the subbands to be used for data transmission. The signal processing may further perform noise averaging to obtain a more accurate estimate of the channel response. As described in detail below, depending on the number of designated subbands used for pilot transmission and the impulse response of the channel, the signal processing may be computationally intensive and require a large number of complex multiplications.
There is therefore a need in the art for techniques to more efficiently derive an estimate of the channel response in a wireless communication system, such as an OFDM system.